This invention relates generally to waste treatment systems and, more specifically, to waste treatment systems whereby the waste is treated using pyrolytic processes.
Modernly, ways of dealing with material that is an undesirable result of an otherwise useful product or process of necessity, have become a serious concern of industrialized nations. As used here, the term "waste" is intended to encompass a wide variety of such material, for example, that material which is considered undesirable or not useful by reason of defects, or damage, or because the material is considered superfluous to, or an unwanted by-product of whatever process or product produced it. Waste is troublesome not only because of the fact it represents something that cannot be used for any beneficial purpose, but because it presents hazards to the environment in terms of the space it takes up and the deleterious effects it has on living organisms. For a considerable period, the disadvantages inherent in waste were largely ignored or, at least afforded little weight when a new process or new product that would produce waste was introduced, the benefits to society that the process or product would bestow being considered paramount. Inevitably, however, the increasing volume of waste and the dangerous conditions presented by it forced more attention to be paid to ways of dealing with the material, such that planning for waste treatment often today is an important consideration in the design of a new process or product.
Pyrolysis long has been known to those skilled in the art of waste treatment as an effective process for reducing the organic components of a variety of compositions of waste material, such as conventional industrial and municipal waste, to products which present no harm to the atmosphere and which can be usein whole or in part to provide a useful source of energy or a material that can be recycled into a product having commercial value.
The pyrolytic process employs high temperature in, most desirably, an atmosphere substantially free of oxygen (for example, in a practical vacuum), to convert the solid organic components of waste to other states of matter, such pyrosylates in a liquid or vapor phase. The solid residue remaining after pyrolysis commonly is referred to as char, but this material may contain some inorganic components, such as metals, as well as carbon components, depending on the nature of the starting waste. The vaporized product of pyrolysis further can be treated by a process promoting oxidation, which "cleans" the vapors to eliminate oils and other particulate matter therefrom, allowing the resultant gases then to be safely released to the atmosphere.
A typical waste treatment system utilizing pyrolysis has an input structure for introducing the waste; a chamber or retort from which air can be purged and in which pyrolysis processing occurs; a feature for raising the temperature inside the chamber; an element that allows the vaporized waste material or "off-gases" to be vented to the environment, which element may or may not include some feature for cleaning or scrubbing the gases; and an assembly through which is discharged the solid or molten residue of the pyrolytic conversion process.
Systems that rely upon pyrolysis often are designed with principal attention being given to the criterion that the system be optimally efficient. For example, to encourage consistent results from the pyrolytic conversion process, various methods and apparatuses commonly are used to pre-treat the waste before it is introduced into the pyrolytic chamber. These include pre-sorting or separating the waste into constituents on the basis of weight, shredding the material to make it of relatively uniform size and perhaps blending it with other pre-sorted material to promote even distribution of the waste as it is introduced into the retort. Several techniques have been employed to reduce the level of moisture in the waste before introducing it into the machine, because the presence of moisture makes the pyrolytic process less efficient. Such techniques include drying by desiccation or through the application of microwave energy.
Other features often are provided to continuously move waste through the treatment unit while the system is being operated, such as a form of conveyance arrangement. Screw conveyors or conveyor belts oriented at an incline have been used to ramp waste material, in units of a defined volume and at a defined rate of flow, up from a storage bin or pre-treatment assembly at the ground level to a charging hopper at the top of the treatment unit through which waste is metered into the pyrolytic chamber. Screw conveyors, auger screws and worm conveyors all have been used to impel waste through the retort while pyrolysis takes place, again, to encourage predictable results from the process.
It is well known that the efficiency of pyrolysis is negatively effected by the presence of oxygen. One of the adverse effects oxygen has is to increase the degree to which the chemical reactions taking place during conversion are explosive, which explosiveness, in turn, increases the turbulence in the chamber and tends to result in the recombination of the released gases with the solid material being processed, making the conversion less complete and thus inefficient. Accordingly, waste treatment systems have been provided with elements that guard against the introduction of oxygen from ambient air surrounding the treatment unit. For example, various air-lock arrangements are known which can be disposed at the point at which waste is introduced into the pyrolytic chamber and also at the point at which the solid residue is discharged from the unit, to insure the retort is kept free of appreciable amounts of oxygen. Vacuum purging elements also have been employed in various stages of the system to periodically remove gases from the pyrolytic chamber to eliminate any air that may have been introduced into it.
The manner in which the retort chamber is supplied with heat energy to sustain pyrolysis also can effect the efficiency with which the process can be carried out. For example, it has been found that uniform application of heat to the outer wall of the retort, through which it is conducted into the interior of the chamber, reduces the risk that the retort will buckle from uneven distribution of high temperatures and tends to encourage a more even distribution of heat and consistency of temperature throughout the chamber, which leads to consistent processing results. System features provided to address even heating have included those directed to the manner in which the primary source of heat energy, commonly fuel gases being combusted in a heating chamber, is arranged with relation to the retort, and the number and placement of fuel gas injection ports, etc.
It further has been known to provide a feature which encourages the efficient use of heat to sustain the pyrolytic process, such as one that allows the recycling of gases that have once been combusted to supply heat energy to the pyrolytic chamber back through the gas injection port, where the gases can be ignited again with a fresh supply of oxygen or air.
Efficiency-promoting elements also can be provided for the processing and recycling of off-gases or vapor pyrosylate. For example, it is known that if a pressure gradient is maintained between the retort and the gas processing arrangement in the direction of the exhaust, the vapor pyrosylate naturally will ten to flow into the cleaning elements. To avoid wasting energy, the cleaned high temperature gases can be used to provide energy to some sort of generating station, such as to heat water in a boiler that supplies a steam generator.
Efficiency considerations then, are of critical importance in designing a waste treatment system or in evaluating the practical value of a system already in existence. For obvious reasons, features that contribute to the safety and reliability with which a system functions also are of primary import. However, there is another criterion that is of great practical concern when a system is being designed or selected, and that is the relative ease with which a treatment unit can be maintained. The maintainability of the system must be considered in assessing for how long a treatment unit can be expected to be continuously operated and for how long it must be shut down to perform refurbishing or repair procedures. The useful life or mean-time-between-failure of individual unit components similarly must be considered in predicting how often operation will have to be disrupted to replace particular components, with attention being given to how easily the component can be accessed and how costly refurbishing, repair or replacement procedures are likely to be in terms of labor and material expenses. Accordingly, gains in efficiency afforded by certain features of prior art waste treatment systems can be overshadowed by the less-than-optimum maintainability of such systems, which leads to frequent, labor-intensive, and complicated maintenance routines that are associated with long periods of system downtime.
What has been needed, therefore, and heretofore has been unavailable is an improved pyrolytic waste treatment system that achieves both optimal maintainability and efficiency, and which nevertheless is safe, reliable and capable of operation with a wide variety of compositions of waste material. The present invention satisfies this need.